1. Field of the Invention
The present invention relates generally to a method for optical proximity correction and, more particularly, to a method for modifying a retarget layout before an optical proximity correction process is applied to the retarget layout.
2. Description of the Prior Art
As integration densities on a semiconductor chips continue to increase at an exponential rate, it is becoming progressively harder to deal with optical effects that arise during the optical lithography process involved in manufacturing the semiconductor chips. These optical effects can cause unwanted distortions in the printed pattern that is generated by the optical lithography process.
To address the problem, an original pattern is often subjected to an optical proximity correction (OPC) operation, which adjusts the pattern to compensate for optical effects. For example, edges in the original pattern may be adjusted to make certain portions of the geometric elements larger or smaller, in accordance with how much additional light exposure is desired at certain points on the substrate. When these adjustments are appropriately calibrated, proximity effects are reduced and overall pattern fidelity is greatly improved
While OPC corrections are commonly applied to the original patterns, it has been pointed out that current OPC techniques cannot guarantee sufficiently process windows for original patterns with increasing integration. Some attempts have been made at improving process windows for specific features by applying a pre-OPC process to the original pattern to get a pre-OPC modified pattern. After the pre-OPC process, the pre-OPC modified pattern may generate new shapes such as assist features on the pattern that provide for improved printability. After the retarget process, a retarget pattern is formed. The retarget process is for improving the process window during the etching process or compensating the loading of the photoresist in a dense region and an isolation region. However, it has been noticed that light intensity varied greatly between dense region and isolation region of the retarget pattern, which may cause defects on the printed pattern.